Fiber optic technology has been recognized for its high bandwidth capacity over longer distances, enhanced overall network reliability and service quality. Fiber to the premises (“FTTP”) (also fiber to the building “FTTB”), as opposed to fiber to the node (“FTTN”) or fiber to the curb (“FTTC”) delivery, enables service providers to deliver substantial bandwidth and a wide range of applications directly to business and residential subscribers. For example, FTTP can accommodate the so-called “triple-play” bundle of services, e.g., high-speed Internet access and networking, multiple telephone lines and high-definition and interactive video applications.
However, utilizing FTTP involves equipping each subscriber premises with the ability to receive optical signals and convert them into electrical signals compatible with pre-existing wiring in the premises (e.g., twisted pair and coaxial). For bi-directional communication with the network, the premises should be equipped with the ability to convert outbound electrical signals into optical signals. In some cases, these abilities are implemented using a passive optical network (“PON”).
Generally speaking, a PON is a point-to-multipoint fiber to the premises network architecture in which un-powered optical splitters are used to enable a single optical fiber to serve multiple subscriber premises, e.g., 16 subscribers, 32 subscribers, etc. A PON generally includes an optical line termination (“OLT”) at the service provider's central office, and a gateway device at each end user location. For example, the premises equipment at each subscriber location may couple to the PON via an optical network unit (“ONU”) (or known as an optical network terminal “ONT”).
In a typical configuration, a single OLT serves multiple ONU/ONTs through a single fiber connection that is split by an optical fiber into fiber connections for each ONU/ONT. Each ONU/ONT includes a “transceiver module” that generally includes a laser and associated driver circuitry and converts electrical signals outgoing from the subscriber equipment into optical signals for upstream transmission to the OLT. Correspondingly, the transceiver module includes an optical receiver to convert downstream optical signals incoming from the OLT into electrical signals for the subscriber equipment. The OLT includes a “transceiver module” having a transmitter for converting electrical signals in a central office to optical signals broadcast downstream on the fiber connection, using an encryption scheme and addressing the data for particular ONU/ONTs. The receivers of the transceiver module must be capable of receiving upstream signals from the ONU/ONTs and converting them to electrical signals.
There are a number of different implementations of a PON. Data Over Cable Service Interface Specification (DOCSIS) PON, or DPON, implements the DOCSIS service layer interface on existing PON infrastructure. DPON for example may implement the DOCSIS Operations Administration Maintenance and Provisioning (OAMP) functionality on existing EPON equipment, making the OLT look and act like a DOCSIS Cable Modem Termination Systems (CMTS) platform. The DOCSIS standards define such things as the format for the modulated digital RF carriers used for communicating between a CMTS and its associated cable modems, the frequencies and RF power levels for transmissions, and the process for requesting and being granted permission to transmit over the cable network. Radio Frequency PON (RF-PON) or Radio Frequency over Glass (RFOG) or Hybrid-Fiber-Coax PON (HFC-PON) or Cable PON, is a type of passive optical network that transmits RF signals that are now transported over copper (principally over a hybrid fiber and coaxial cable) over PON.
PON transmissions are examples of “burst” transmissions, in which packet data is sent from the OLT to the ONU/ONTs and from the ONU/ONTs to the OLT in bursts of data. Upstream bursts can come from each of the ONU/ONTs and are sent using a multiple access protocol, such as time division multiple access (TDMA). The bursts contain training symbols, preamble, and payload data.
OLTs are examples of burst mode receivers. As each “burst” signal is received from an ONU/ONT, the OLT must quickly synchronize to the clock of the burst signal and then decode the data within the burst. As networks such as Gigabit PONs (GPONs) move to higher throughput, data is compacted and the spacing between bursts is shortened. This means that OLTs must be able to more quickly synchronize with the clock of a received burst signal and decode the corresponding data. Add to this the increasing dynamic range required of OLTs. As the distance between an OLT and the various, corresponding ONU/ONTs may vary, as well as the number of ONU/ONTs per OLT, the OLT must be able to handle a wide range of power levels on received burst signals. Both of these demands, increased network throughput and high dynamic range, constrain burst mode receives design.
Although the OLT operates as a linear, low noise optical receiver, current design implementations are limited in handling signal bursts. At the start of a transmission, the received optical power quickly transitions (few hundred nanoseconds) from an idle power that is near zero to an on state which consists of an average power that is maintained for the duration of the burst plus an RF modulation component. The sudden increase in the received optical power can vary depending on the proximity of the transmitting ONU/ONT, and thus can change from burst to burst as the distance to the OLT can vary from ONU/ONT to ONU/ONT. In conventional systems, the sudden increase results in an overload condition that disrupts the amplifier bias voltages and currents. It is therefore desired to develop optical receiver designs that increase the overload capacity for received burst signals, in particular those receivers used in PON type configurations.